The 2006 Duramax did have a 5yr/100K mile warranty, but for the engine only. So glowplugs would be covered. GM changed the warranty to cover the complete powertrain starting with the 2007 model year.
Only for model year 2007 and newer is the Allison covered for 5yr/100K. For 2006 and older models the Allison was covered by the 36mo/36K mile bumper to bumper warranty. Unless you bought a used GM Certified truck from a GM dealer after Jan 1, 2007, in which case GM extended the complete powertrain warranty to 5yr/100K warranty for the GM certified vehicles in an attempt to increase used vehicle sales at their dealers.